Tri-color white light led lamp

ABSTRACT

The invention relates to a tri-color lamp for generating white light comprising a phosphor composition comprising a phosphor of general formula (Ba 1-x-y-z- Sr x Ca y ) 2 SiO 4 :Eu z , wherein 0≦x≦1, 0≦y≦1 and 0&lt;z&lt;1. The phosphor composition may also comprise a red phosphor. The two phosphors can absorb radiation emitted by a light emitting diode, particularly a blue LED. This arrangement provides a mixing of three light sources—light emitted from the two phosphors and unabsorbed light from the LED. The invention also relates to an alternative to a green LED comprising a single green phosphor of general formula (Ba 1-x-y-z- Sr x Ca y ) 2 SiO 4 :Eu z , wherein 0≦x≦1, 0≦y≦1 and 0&lt;z&lt;1, that absorbs radiation from a blue LED. A resulting device provides green light of high absorption efficiency and high luminous equivalent values.

FIELD OF INVENTION

The present invention relates to a tri-color lamp. The lamp comprises aphosphor composition and a light emitting diode for an excitation energysource. In particular, the lamp employs a blue LED and a mixture of redand green phosphors for the production of white light.

BACKGROUND OF THE INVENTION

There is an ongoing need to generate new phosphor compositions toimprove efficiency and color quality in luminescent devices,particularly in the production of white light. Phosphors are luminescentmaterials that can absorb an excitation energy (usually radiationenergy) and store this energy for a period of time. The stored energy isthen emitted as radiation of a different energy than the initialexcitation energy. For example, “down-conversion” refers to a situationwhere the emitted radiation has less quantum energy than the initialexcitation radiation. Thus, the energy wavelength effectively increases,and this increase is termed a “Stokes shift”. “Up-conversion” refers toa situation where the emitted radiation has greater quantum energy thanthe excitation radiation (“Anti-Stokes shift”).

Improvements in efficiency and color quality in phosphor-based devicesare constantly being developed. “Efficiency” relates to the fraction ofphotons emitted with respect to a number of photons initially providedas excitation energy. Inefficient conversion results when at least aportion of the energy is consumed by non-radiative processes. Color“quality” can be measured by a number of different rating systems.“Chromaticity” defines color by hue and saturation. “CIE” is achromaticity coordinate system developed by Commission Internationale del'Eclairage (International commission on illumination). The CIEChromaticity Coordinates are coordinates that define a color in “1931CIE” color space. These coordinates are defined as x, y, z and areratios of the three standard primary colors, X, Y, Z (tristimulusvalues), in relation to the sum of the three tristimulus values. A CIEchart contains a plot of the x, y and z ratios of the tristimulus valuesversus their sum. In the situation where the reduced coordinates x, y, zadd to 1, typically, a two-dimensional CIE (x, y) plot is used.

White-like colors can be described by a “correlated color temperature”(CCT). For example, when a metal is heated, a resulting light is emittedwhich initially glows as a red color. As the metal is heated toincreasingly higher temperatures, the emitted light shifts to higherquantum energies, beginning with reddish light and shifting to whitelight and ultimately to a bluish-white light. A system was developed todetermine these color changes on a standard object known as a blackbodyradiator. Depending on the temperature, the blackbody radiator will emitwhite-like radiation. The color of this white-like radiation can then bedescribed in the CIE chromaticity chart. Thus, the correlated colortemperature of a light source to be evaluated is the temperature atwhich the blackbody radiator produces the chromaticity most similar tothat of the light source. Color temperature and CCT are expressed indegrees Kelvin.

A “color rendering index” (CRI) is established by a visual experiment.The correlated color temperature of a light source to be evaluated isdetermined. Then eight standard color samples are illuminated first bythe light source and then by a light from a blackbody having the samecolor temperature. If a standard color sample does not change color,then the light source has a theoretically perfect special CRI value of100. A general color rendering index is termed “Ra”, which is an averageof the CRIs of all eight standard color samples.

Older white lamps involved emission of light over a broad wavelengthrange. It was then discovered that a white-like color can be simulatedby a mixture of two or three different light colors, where each emissioncomprised a relatively narrow wavelength range. These lamps affordedmore control to manipulate the white color because emissive properties(emission energy and intensity) of the individual red, green and bluelight sources can be individually tailored. This method thus providedthe possibility of achieving improved color rendering properties.

An example of a two-color lamp comprises one phosphor and an excitationenergy source. Light emitted by the phosphor combines with unabsorbedlight from the excitation source to produce a white-like color. Furtherimprovements in fluorescent lamps involved three different light colors(i.e. a tri-color lamp) resulting in white light at higher efficiencies.One example of a tri-color lamp involved blue, red and greenlight-emitting phosphors. Other previous tri-color lamps comprised acombination of light from two phosphors (a green and red phosphor) andunabsorbed light from a mercury plasma excitation source.

Previous tri-color lamps involving a mercury plasma excitation source,however, suffer many disadvantages including: (1) a need for highvoltages which can result in gaseous discharge with energetic ions; (2)emission of high energy UV quanta; and (3) correspondingly lowlifetimes. Thus, there is an ongoing need for devices that overcomethese deficiencies.

WO 01/24229 discloses to a tri-color lamp for generating white light. Inparticular, WO 01/24229 relates to a phosphor mixture comprising twophosphors having host sulfide materials that can absorb radiationemitted by a light emitting diode, particularly a blue LED. Thisarrangement provides a mixing of three light sources—light emitted fromthe two rare earth ion, to allow matching of the phosphors in relationto the LED emitted radiation. Power fractions of each of the lightsources can be varied to achieve good color rendering. WO 01/24229 alsorelates to an alternative to a green LED comprising a single greenphosphor that absorbs radiation from a blue LED. A resulting deviceprovides green light of high absorption efficiency and high luminousequivalent values.

There remains a continued challenge to uncover phosphor compositions andmixtures of these compositions to provide improved properties, includingimproved efficiency, color rendering (e.g. as measured by high colorrendering indices) and luminance (intensity), particularly in atri-color, white lamp.

It has been observed that photoluminescent phosphor compounds havecharacteristic ranges of luminescent quenching temperatures. That is,when the phosphor is excited into luminescence as for example bysubjecting the same to radiation from a source of ultraviolet light, theintensity of the luminescence will gradually decrease as the temperatureof the phosphor is raised through a specified range of temperatures. Forexample, the compound Zn.80 Cd.20 S:AgCl will gradually quench frombright green, to dull green, to green gray, to gray, to black, as thetemperature is increased from 95° C. to 105° C. The temperature rangesof the compounds provided by the afore described basic phosphor systemare found to be determined by the composition of their anioniccomponent. Phosphor compounds may be therefore made to have differentquenching temperature ranges as well as different color characteristics.The thermal quenching at higher temperatures reduces the LED efficiency,in particular at high chip temperatures. Therefore it is desirable toreplace sulfide phosphors by non -sulfide phosphors with a higherthermal quenching temperature.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a device comprising a lightemitting diode, for emitting a pattern of light. The device furthercomprises a phosphor composition comprising a phosphor of generalformula (Ba_(1-x-y-z-)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1and 0<z<1.

The phosphor comprises as a host an alkaline earth silicate lattice anddivalent Europium as a dopant and the composition is positioned in thelight pattern.

Such a device shows high efficiency, particularly at chip temperatureabove 150° C., because of the high quenching temperature of such aphosphor.

Especially, when the phosphor is comprised in a thin film layer the highquenching temperature of the phosphor is of advantage.

Another aspect of the present invention provides a light emitting devicecomprising a LED for emitting a pattern of light and a phosphorcomposition comprising a mixture of a first phosphor and a secondphosphor. The first phosphor has the general formula(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1and comprises a host alkaline earth silicate lattice and divalentEuropium as a dopant and is capable of being excited by a light emittingdiode. The second phosphor is a red-emitting phosphor.

Another aspect of the present invention provides a light emitting devicewherein the phosphor composition comprises a green phosphor of generalformula (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and0<z<1, and a red phosphor, wherein the red phosphor is selected from thegroup of (Sr_(1-x-y)Ba_(x)Ca_(y))S:Eu wherein:

0≦x<1 and 0≦y≦1;

CaS:Ce,Cl; Li₂Sr SiO₄:Eu; (Sr_(1-x)Ca_(x))SiO₄:Eu wherein 0≦x<1;

(Y_(1-x)Gd_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce wherein 0≦x<1 and 0≦y<1 and

(Sr_(1-x-y)Ba_(x)Ca_(y))₂Si₅N₈:Eu wherein 0≦x<1 and 0≦y<1. The emissionspectrum of such a phosphor composition has the appropriate wavelengthsto obtain together with the blue light of the LED a high quality whitelight with good color rendering at the required color temperature.

Another aspect of the present invention provides a phosphor compositioncomprising a phosphor of general formula(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:EU_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1and comprising a host alkaline earth silicate lattice and divalentEuropium as a dopant. A phosphor of general formula(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1absorbs photons efficiently, especially photons of a blue LED operatingat 450 nm and shows high quantum efficiency upon excitation of suchwavelength. Such phosphor shows high quenching temperature of TQ 50% at230° C.

Other advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings, which areschematic and which are not intended to be drawn to scale. In thefigures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forthe purposes of clarity, not every component is labeled in every figure,nor is every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a tri-color lamp comprising a two-phosphormixture of (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and Sr₂Si₅N₈:Eupositioned in a pathway of light emitted by an LED.

FIG. 2 shows an overlay of emission and excitation spectrum of(Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) upon excitation by a blue LED at 460nm;

FIG. 3 shows a simulation of spectra of a mixture of(Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and Sr₂Si₅N₈:Eu upon excitation by ablue LED at 450 nm at different color temperatures; and

FIG. 4 shows spectra of LEDs comprising a mixture of(Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and SrS:Eu and a blue LED emittingat 450 nm at different color temperatures;

FIG. 5 shows the reflection spectra of(Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) without an SiO₂ coating and with anSiO₂ coating.

DETAILED DESCRIPTION

The invention relates, in part to the discovery that a tri-color lampemploying specific green and red phosphors excitable by a common lightemitting diode (LED) can achieve white light at higher efficiencies withsuperior color rendering over previous fluorescent lamps and are not aptto substantial quenching at chip temperature above 150° C. and up to230° C.

An advantageous feature of the present invention involves the use of anLED as an excitation source. An LED has a p-n junction between dopedsemiconductor regions. Upon application of a current, there can existsufficient energy to allow electrons and holes to cross the p-n junctionsuch that a resulting recombination of electrons and holes causesemission of radiation. Advantages of LEDs over other excitation energysources include small size, low power consumption, long lifetimes andlow amounts of thermal energy emitted. In addition, LEDs have smalldimensions that allow miniaturization of devices.

In one embodiment, the common LED is a blue LED. The use of blue lightas an excitation radiation over other light sources has been found to beparticularly advantageous in that conversion efficiency to visible lightis higher. In one embodiment each phosphor is capable of being excitedby a common LED which emits radiation at a wavelength from about 450 nmto about 480 nm. It has been found that color rendering can decrease atexcitation energies below 450 nm whereas absorption by the phosphorsdecreases at excitation energies greater than 480 nm. An example of ablue LED that emits radiation in the above-mentioned energy ranges is a(In,Ga)N diode.

A blue light source can provide inherent advantages over UV excitationsources in that power efficiency is increased for red and greenphosphors excited by blue light. The present phosphor materialsgenerally require lesser Stokes shifts than phosphors of the previousdevices. For example, certain tri-color prior art fluorescent lampsincorporate mercury plasmas which provide a UV emission centered atapproximately 4.9 eV. This UV light excites blue, red and greenphosphors such that resulting emission spectra show maximum intensitiesat energies of approximately 2.8 eV (unabsorbed light), 2.3 eV (green)and 2.0 eV (red) respectively. Significant Stokes shifts are obviouslyinvolved in this situation. Power efficiency, however, is limited byquantum deficit, which is the difference of the quantum energies ofexciting and emitted quanta. Thus, for the example described above,power efficiency of the green light is, on average, (4.9 eV-2.3 eV)/4.9eV=53%. In contrast, green (2.3 eV) and red {2.0 eV) phosphors excitedby a blue LED with an emission of about 2.7 eV (˜460 nm) exhibit smallerStokes shifts and quantum deficits, and accordingly power efficiency isgreater.

Phosphors comprise a host lattice and dopant ions. Typically, the hostmaterial has an inorganic, ionic lattice structure (a “host lattice”) inwhich the dopant ion replaces a lattice ion. The dopant is capable ofemitting light upon absorbing excitation radiation. Ideal dopantsstrongly absorb excitation radiation and efficiently convert this energyinto emitted radiation. If the dopant is a rare earth ion, its absorband emits radiation via 4f-4f transitions, i.e. electronic transitionsinvolving f-orbital energy levels. While f-f transitions arequantum-mechanically forbidden resulting in weak emission intensities,it is known that certain rare earth ions, such as divalent Europiumstrongly absorb radiation through allowed 4f-5df transitions (viad-orbital/f-orbital mixing) and consequently produce high emissionintensities.

The emissions of rare earth dopants, such as divalent Europium can beshifted in energy depending on the host lattice in which the dopant ionresides. Thus, this aspect of the invention lies, in part, in thediscovery that certain rare earth dopants efficiently convert blue lightto visible light when incorporated into an appropriate host material.The phosphor according to the invention comprises a host lattice, whichis a earth alkaline silicate, i.e. a lattice which includes silicate andearth alkaline metal ions.

Another advantageous feature of the present invention is to provide aphosphor mixture excitable by one common blue energy source of arelatively narrow linewidth emit light at two different energy ranges(e.g. red and green). Strategies to provide appropriate phosphors aredisclosed here. In one embodiment, the dopant is the same in the firstand second phosphor. The red and green emissions of the two phosphorscan be tuned by selecting an appropriate host material. The red phosphoris selected from the group consisting of (Sr_(1-x-y)Ba_(x)Ca_(y))S:Euwherein 0≦x<1 and 0≦y<1;

CaS:Ce,Cl; Li₂Sr SiO₄:Eu; (Sr_(1-x)Ca_(x))SiO₄:Eu wherein 0≦x<1;(Y_(1-x)Gd_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce wherein 0≦x<1 and 0≦y<1 and(Sr_(1-x-y)Ba_(x)Ca_(y))₂Si₅N₈:Eu wherein 0≦x<1 and 0≦y<1.

FIG. 2 is an overlay of emission spectra from two different phosphorsi.e. (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and Sr₂Si₅N₈:Eu being excitedby one common {In,Ga)N LED at 450 nm. The phosphors are provided as amixture in an encapsulant. The spectra have been normalized to anintensity of unity.

FIG. 3 is an overlay of emission spectra from two different phosphorsi.e. (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and SrS:Eu being excited by onecommon {In,Ga)N LED at 450 nm. The phosphors are provided as a mixturein an encapsulant. The spectra have been normalized to an intensity ofunity.

Preferably the amount of dopant present in the host lattice is from 0.01mol % to 8 mol %.

In the operation of the embodiment of the present invention radiation ofultraviolet rays from the LED chip onto the phosphor layer excites thephosphors and produces a bright and uniform luminescence over the entiresurface off the layer.

In one embodiment, the device is a lamp. In one embodiment, the lampemits white light. In this embodiment, the first phosphor is a greenphosphor of general formula (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and thesecond phosphor is a red phosphor of general formula Sr₂Si₅N₈:E, wherethe white color is achieved by effectively mixing the green and redlight with unabsorbed blue light provided by the LED.

The white light lamp of the present invention is particularlyadvantageous in that the color rendering properties are far superior tothose of any previous white lamps. In one embodiment, the lamp iscapable of emitting radiation having a color rendering index, Ra of atleast about 60 at a color temperature from about 2700 K to about 8000 K,preferably an Ra of at least about 70, more preferably an Ra of at leastabout 80 and even more preferably an Ra of at least about 90. In apreferred embodiment, the lamp generates a CRI, Ra of greater than 70for CCT values of less than 6000 K.

FIG. 2 shows a simulation of emission spectra of a tri-color lampcomprising a mixture of (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) andSr₂Si₅N₈:Eu excited by a blue (In,Ga)N LED at 460 nm. Emission intensityor radiance is indicated in the ordinate axis. This system exhibitssuperior color rendering properties over a wide range of colortemperatures. It is a feature of the present invention to mix respectiveamounts of the phosphors and to illuminate these phosphors with a blueLED to closely match these desired simulated spectra.

By varying optical properties of each source of light in the device, thedevice can be designed to have desired characteristics depending on aparticular application. For example, certain devices may be required togenerate light of high intensity and only adequate color rendering isneeded, whereas other applications may require high color renderingproperties, at the expense of efficiency. Alternatively, color renderingcan be sacrificed for higher efficiency. For example, a 50% increase inefficiency can be achieved by decreasing Ra down to about 60. Suchproperties can be varied by changing relative power fractions of eachlight source. A “power fraction” is the fraction of light from eachsource that provides the final light color. Power fractions can bevaried by, for example, changing a relative amount of phosphor materialpresent in the device or varying dopant concentration.

It is understood that the phosphor composition can comprise more thantwo phosphors so long as optimal color rendering properties areachieved.

In one embodiment, the device further comprises a polymer forencapsulating the phosphor composition. In this embodiment, the phosphormixture should exhibit high stability properties in the encapsulant.Preferably, the polymer is optically clear to prevent significant lightscattering. In one embodiment, the polymer is selected from the groupconsisting of epoxy and silicone resins. A variety of polymers are knownin the LED industry for making 5 mm LED lamps. Encapsulation can beperformed by adding the phosphor mixture to a liquid that is a polymerprecursor. For example, the phosphor mixture can be a powder.Introducing phosphor particles into a polymer precursor liquid resultsin formation of a slurry (i.e. a suspension of particles ). Uponpolymerization, the phosphor mixture is fixed rigidly in place by theencapsulation. In one embodiment, both the composition and the LED areencapsulated in the polymer.

The use of phosphors with general formula(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1is especially advantageous if the phosphor composition is applied as athin film or in a small volume, because they are not sensitive to highertemperatures, which result in thin layers due to heat generated by theStokes shift together with strong absorption and consequently very smalllight penetration depth.

The phosphor comprising composition may be fabricated by eventually dryblending phosphors in a suitable blender and then assign to a liquidsuspension medium or the individual phosphor or phosphors may be addedto a liquid suspension, such as the nitrocellulose/butylacetate binderand solvent solution used in commercial lacquers. Many other liquidsincluding water with a suitable dispersant and thickener or binder suchas polyethylene oxide can be used. The phosphor containing compositionis painted or coated or otherwise applied on the LED and dried.

Otherwise the phosphor or phosphors can be combined with a suitablepolymer system, such as polypropylene, polycarbonate, orpolytetrafluoroethylene, to a phosphor composition, which is then coatedor applied to the LED and dried, solidifies, hardeners, or cured. Theliquid polymer system may optionally be UV cured or room temperaturecured to minimize any heat damage to the LED.

Otherwise a clear polymer lens made of suitable plastic such aspolycarbonate or other rigid transparent plastic is molded over the LED.Lens may be further coated with anti-reflective layers to facilitatelight to escape the device.

Although the role of phosphor grain size (mean diameter of phosphorparticles) is not completely understood, weight fractions may changedepending on a particular grain size. Preferably, grain sizes are lessthan 15 μm, and more preferably, less than 12 μm, to avoid clogging ofdevices which dispose the phosphors. In one embodiment, the grain sizeof each phosphor type varies. In certain specific embodiments, the grainsize of (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and0<z<1 is less than about 10 μm. Other devices, however, can be preparedwith larger grain sizes.

Athough unabsorbed light emitted from the LED contributes to colorrendering, unabsorbed light can sometimes escape without mixing withlight emitted from the phosphors, resulting in a reduced overallefficiency of the device. Thus, in one embodiment, the LED andcomposition are positioned within a reflector cup. A reflector cup canbe any depression or recess prepared from a reflecting material. Bypositioning the LED and phosphor particles in a reflector cup,unabsorbed/unmixed LED-emitted light can be reflected either back to thephosphor particles to eventually be absorbed, or mixed with lightemitted from the phosphors.

FIG. 5 shows a schematic view of the device of the present invention.The device comprises LED 1. LED 1 is positioned in a reflector cup 2.LED 1 emits light in a pattern. A phosphor composition 4,5, ispositioned in the pattern. The phosphor composition is embedded in aresin 3. In this example, reflector cup 2 can modify light pattern iflight is reflected into a space not previously covered by the initiallight pattern (e.g. in the case of a parabolic reflector). It isunderstood that one of ordinary skill in the art can provide reflectorcup 2 in any shape that optimizes reflection of light back to phosphorcomposition 4,5, or optimizes positioning of LED 1 to provide a lightpattern for efficient conversion. For example, the walls of reflectorcup 2 can be parabolic.

Another aspect of the present invention provides an alternative designto a green LED. Green LEDs have been developed only recently. Currentgreen LEDs, however, are notoriously more inefficient than blue LEDs. Inaddition, emitted radiation from green LEDs exhibits wavelength shiftswith an increase in temperature, which is an undesired characteristic.

Thus, this aspect of the present invention provides a device comprisinga green phosphor and a blue light emitting diode, for providing anexcitation radiation to the phosphor. By taking advantage ofdown-conversion, the blue light can be converted to green light via thegreen phosphor. This device is comparable to a green LED yet eliminatesthe disadvantages of green LEDs, such as providing comparableefficiencies to blue LEDs and minimizing radiation energy shifts withincreasing temperature. In one embodiment, the green phosphor is(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z) wherein 0≦x≦1, 0≦y≦1 and 0<z<1.

High luminous equivalents values of 575 lm/W can be achieved with thisphosphor at a maximum wavelength of about 535 nm, which is far superiorto any other green LED or alternative. High absorption of the excitationradiation is preferred to eliminate a significant amount of blueradiation which may spoil efficiency and/or color saturation.

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the examples below. Thefollowing examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

EXAMPLE 1

Preparation of (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02)

15.00 9 (76.01 mmol) BaCO3, 11.22 (76.01 mmol) SrCO3, 0.546 (1.55 mmol)EU203, and 4.80 9 (79.90 mmol) SiO2 are suspended in ethanol. Thesolvent is slowly evaporated under stirring and NH4Cl is subsequentlyadded to the dried powder. After the material has been thoroughly groundin an agate mortar, the material is filled into alumina crucibles andfired at 600° C. for 0.5 h. The powder is ground again and the obtainedpowder is annealed at 1100 to 1200° C. for 4 h under N2/H2. Cooling downto room temperature occurs under N2/H2. Finally, the powder is milledand sieved through a 36 μm sieve.

The spectroscopic properties of (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02),prepared this way, are: QE_(460 nm) of 70%, Abs_(460 nm) of 80%, LE of483 [lm/W] and colour point x=0.24, y=0.65.

EXAMPLE 2

White LED comprising (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) andSr₂Si₅N₈:Eu.

A phosphor blend comprising (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) andSr₂Si₅N₈:Eu is suspended in silicone monomer oil and a droplet of thesuspension is deposited onto an InGaN die. A catalyst is added to thesilicone monomer to start the polymerization process, resulting inhardening of the silicone. Finally, the LED is sealed with a plasticcap.

EXAMPLE 3

White LED comprising (Ba_(0.49)Sr_(0.49))₂SiO₄:EU_(0.02) and SrS:Eu Aphosphor blend comprising (Ba_(0.49)Sr_(0.49))₂SiO₄:Eu_(0.02) and SrS:Euis suspended in silicone monomer oil and a droplet of the suspension isdeposited onto an InGaN die. A catalyst is added to the silicone monomerto start the polymerization process, resulting in hardening of thesilicone. Finally, the LED is sealed with a plastic cap.

1. A light emitting device comprising a LED for emitting a pattern oflight and a phosphor composition comprising a phosphor of generalformula (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and0<z<1.
 2. The light emitting device according to claim 1, wherein thelight emitting device has an emission at a wavelength from 450 nm to 480nm.
 3. The light emitting device according to claim, wherein thephosphor composition is comprised in a thin film layer.
 4. The lightemitting device according to claim 1, wherein the LED is a blue-emittingLED.
 5. The light emitting device according to claim 1, wherein thephosphor composition comprises a green phosphor of general formula(Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0≦x≦1, 0≦y≦1 and 0<z<1and a red phosphor.
 6. The light emitting device according to claim 5,wherein the red phosphor is selected from the group of(Sr_(1-x-y)Ba_(x)Ca_(y))S:Eu wherein 0≦x<1 and 0<y≦1; CaS:Ce,Cl; Li₂SrSiO₄:Eu; (Sr_(1-x)Ca_(x))SiO₄:Eu wherein 0≦x<1;(Y_(1-x)Gd_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce wherein 0≦x<1 and 0≦y<1 and(Sr_(1-x-y)Ba_(x)Ca_(y))₂Si₅N₈:Eu wherein 0≦x<1 and 0≦y<1.
 7. The lightemitting device according to claim 1, wherein the device is a lamp. 8.The light emitting device according to claim 1, wherein the device is atraffic sign.
 9. A phosphor composition comprising a phosphor of generalformula (Ba_(1-x-y-z)Sr_(x)Ca_(y))₂SiO₄:Eu_(z), wherein 0 x 1, 0 y 1 and0<z<1.